Thirty years ago China began launching ambitious reforestation projects, aiming to undo some of the massive environmental damage caused by years of clear cutting. Each year the country adds around 2 million hectares of new trees, with no sign of slowing. China plans to add 40 million hectares of planted land between 2005 and 2020.
A team of Chinese and French scientists have asked what afforestation, the process of adding new trees, means for land surface temperatures. They present their findings in PNAS Early Edition. In some areas, they find, afforestation leads to an overall cooling effect, a plus for combatting climate change. In other areas, particularly in dry regions, afforestation leads to a net warming effect.
“These results suggest it is necessary to carefully consider where to plant trees to achieve potential climatic benefits in future afforestation projects,” write Shu-Shi Peng and co-authors.
Generally forests absorb more sunlight than grasslands, which have a higher albedo or reflection coefficient. This tends to increase the amount of heat on the land. However this effect tends to be offset by evaporative cooling. The cooling leads to an average decrease of around 1.1 °C during the day. Forested areas tend to have warmer temperatures at night however, an effect which increases with latitude and dryness. Wetter regions, the authors suggest, are better places to plant new forests.
The study drew on NASA satellite measurements of land surface temperature to compare reforested areas to adjacent crop and grasslands. This is the first study of its kind and is relatively simple in its comparisons. Future studies, the authors hope, will examine “the full range of climatic effects of afforestation by combining energy fluxes from eddy flux towers, satellite observations, and land surface models coupled with climate models.”